Sedimentary gold-bearing ores containing indigenous organic carbonaceous material and gold-bearing sulfidic minerals are notoriously refractory to standard cyanidation treatment for the recovery of their gold content. Investigation into the cause of this problem has indicated that the carbonaceous materials comprises active carbon and long-chain organic compounds. The active carbon appears to absorb the gold cyanide complex [Au(CN).sub.2 -] from cyanide leaching solutions and the long-chain organic compounds appears to form stable complexes with the gold. In addition, some of these ores also contain gold-bearing sulfides. The sulfides contain gold either as a mechanical inclusion, or as atoms of gold included in the sulfide crystal lattice.
In order to overcome this sequestering of the gold and to render the gold component of the ore more amenable to standard cyanidation treatment it has heretofore been proposed that the ore be subjected to a preliminary oxidation treatment to oxidize the carbonaceous, sulfidic mineral materials, and as much of the carbonaceous mineral materials as can be oxidized. In experiments conducted by the United States Bureau of Mines, alkaline slurries of carbonaceous gold-bearing ores were subjected to a preliminary oxidation with a wide variety of reagents including ozone, sodium hypochlorite, calcium hypochlorite, permanganates, perchlorates, chlorates and oxygen prior to subjecting the oxidized ore to standard cyanidation treatment to extract the gold content therefrom. Of these treatments, the hypochlorites appeared to be the most effective, although it was found that an elevated temperature was required to obtain satisfactory results. At room temperature (about 70.degree. F.) the hypochlorite treatment required several days to make the gold content of the ore available for recovery by cyanidation while temperatures above about 140.degree. F. caused premature decomposition of the hypochlorites. Consequently, it was determined that a temperature within the range of about 122.degree. to 140.degree. F. was required to render the gold component of the ore amenable to standard cyanidation treatment.
In U.S. Pat. No. 3,846,124 to Wilbur J. Guay it was shown that the recoverability, by standard cyanidation, of the gold content of carbon-containing sedimentary gold bearing ores is increased by subjecting the ore to a preliminary oxidation and chlorination treatment in which chlorine gas is introduced into an aqueous slurry of the ground ore having a pH of about 8 to the extent that the slurry will absorb the chlorine, the thus treated slurry being maintained at the chlorination treatment temperature of about 70.degree. to 85.degree. F. for at least 6 hours. The oxidized ore is then subjected to standard cyanidation to extract the gold content therefrom with gold recoveries in the order of 75% or more of the gold content of the ore.
In U.S. Pat. No. 4,038,362 to Wilbur J. Guay it was shown that the recoverability, by standard cyanidation, of the gold content of sedimentary gold-bearing ores containing organic carbonaceous material and gold-bearing pyrite, or other gold-bearing sulfides, is increased and the cost of the pretreatment greatly reduced by subjecting the ore to a two-stage preliminary oxidation and chlorination treatment. In this process an aqueous slurry of the ore is first heated to about 167.degree. to 212.degree. F., and air or oxygen is introduced into the heated slurry to oxidize and eliminate a substantial portion of the carbonaceous material and oxidizable sulfides in the slurried ore. The slurry is then cooled to about 70.degree. to 85.degree. F., and chlorine gas is introduced into the slurry at a pH of 8 or higher to substantially complete the oxidation and chlorination of the carbonaceous and sulfide content of the slurried ore. The oxidized ore is then subjected to conventional cyanidation to recover the gold content thereof.
I have now discovered that the recoverability of gold from the ore is enhanced by the addition of acid to the aqueous slurry of the ore to reduce the pH of the slurry to 6.0 or less following the first or oxidation stage and prior to the chlorination stage of the preliminary two-stage treatment of the ore. I have also discovered that the acidification of the slurry permits the chlorination stage of the preliminary treatment of the ore to be carried out at a temperature of from 70.degree. to 125.degree. F., as compared to 70.degree. to 85.degree. F. previously required, and I have also found that the acidification of the ore slurry significantly increases the rate at which sodium hypochlorite will react with carbonaceous and sulfidic constituents of the ore at temperatures within this range so as to make it feasible to use sodium hypochlorite as an alternative to chlorine in the chlorination stage of the process.